1. Field of the Invention
The present invention relates to a magnetoresistance effect element for reading the magnetic field intensity of magnetic recording media as signals, especially a magnetoresistance effect element capable of reading a small magnetic field change as a greater electrical resistance change signals, and a magnetoresistance device such as a magnetoresistance effect type head or the like using the same.
2. Prior Art
There are recently growing demands for increasing the sensitivity of magnetic sensors and increasing the density of magnetic recording, and active research works have been devoted for the development of magnetoresistance effect type magnetic sensors (hereinafter referred to as MR sensors) and magnetoresistance effect type magnetic heads (hereinafter referred to as MR heads). Both MR sensors and MR heads are designed to read out external magnetic field signals on the basis of the variation in resistance of a reading sensor portion formed of magnetic material. The MR sensors have an advantage that a high sensitivity can be obtained and the MR heads have an advantage that a high output can be obtained in high density magnetic recording because the reproduced output does not depend on the relative speed of the sensors or heads to the recording medium.
However, conventional MR sensors which are formed of magnetic materials such as Ni.sub.0.8 Fe.sub.0.2 (Permalloy), NiCo or the like have a small magnetoresistance change .DELTA.R/R which is about 1 to 3% at maximum, and thus these materials have insufficient sensitivity as the reading MR head materials for ultrahigh density recording of the order of several giga bit per square inch (GBPI) or more.
Attention has been recently paid to artificial superlattices having the structure in which thin films of metal having a thickness of an atomic diameter order are periodically stacked, because their behavior is different from that of bulk metal. One of such artificial superlattices is a magnetic multilayer film having ferromagnetic metal thin films and antiferromagnetic metal thin films alternately deposited on a substrate. Heretofore, magnetic multilayer films of iron-chromium and cobalt-copper types have been known. Among these materials, the iron-chromium (Fe/Cr) type was reported to exhibit a magnetoresistance change which exceeds 40% at an extremely low temperature (4.2K) (see Phys. Rev. Lett., Vol. 61, p2472, 1988). However, this artificial superlattice magnetic multilayer film is not commercially applicable if it is left as it is because the external magnetic field at which a maximum resistance change occurs (that is, operating magnetic field intensity), is as high as ten to several tens of kilo-oersted. Additionally, there have been proposed artificial superlattice magnetic multilayer films of Co/Ag, which require too high operating magnetic field intensity.
Under these circumstances, a new structure which is called as a spin valve film is proposed. In this structure, two NiFe layers are formed through a non-magnetic layer, and an FeMn layer is further formed so as to be adjacent to one of the NiFe layers. In this case, since the FeMn layer and the NiFe layer adjacent thereto are directly exchange-coupled to each other, the direction of the magnetic spin of this NiFe layer is fixed in the range of several tens to several hundreds Oe in magnetic field intensity. On the other hand, the direction of the magnetic spin of the other NiFe layer is freely varied by an external magnetic field. As a result, there can be achieved a magnetoresistance change rate (MR ratio) in a small range of 2 to 5%, which corresponds to the degree of coercive force of the NiFe layer. In addition, the following papers have been published.
a. Physical Review B, 43 (1991) 1297
Si/Ta(50)/NiFe(60)/Cu(20)/NiFe(45)/FeMn(70)/Ta(50) [parenthesis represents film thickness (.ANG.) of each layer] is reported to exhibit that its MR ratio sharply rises up to 5.0% at an applied external magnetic field of 10 Oe.
b. Journal of Magnetism and Magnetic Materials, 93 (1991) 101
Si/Ta(50)/NiFe(60)/Cu(25)/NiFe(40)/FeMn(50)/Cu(50) is reported to exhibit that its MR ratio sharply rises up from 0 to 4.1% at an applied external magnetic field of 0 to 15 Oe.
c. Physical Review B, 45 (1992) 806
The temperature characteristics and MR characteristics when the magnetic layer thickness is varied and Co, NiFeNi, etc. are used as the magnetic layer are analyzed on the basis of the results of the above papers a and b.
d. Japanese Journal of Applied Physics, 32 (1993) L1441
The MR ratio is reported when the multilayer structure is adopted in the above papers a and b. In this multilayer structure, the structure of NiFe(60)/Cu(25)/NiFe(40)FeMn(50)is laminated so as to sandwich Cu therebetween.
Furthermore, the following publications are made public.
e. Japanese Laid-open Patent Application No. Hei-2-61572 (U.S. Pat. No. 4,949,039)
It is described that a large MR effect can be obtained by forming ferromagnetic thin films through a non-magnetic intermediate layer so as to be arranged in anti-parallel to each other. In addition, it describes a structure in which antiferromagnetic material is disposed adjacently to one of the ferromagnetic layers.
f. Japanese Laid-open Patent Application Hei-5-347013
A magnetic recording and reproducing device using a spin valve film is described. Particularly, it is disclosed that nickel oxide is used for an antiferromagnetic film.
In such a spin valve magnetic multilayer film, the MR ratio is lower than the structure of Fe/Cr, Co/Cu, Co/Ag or the like, however, the MR curve varies sharply at an applied magnetic field below several tens Oe, so that it is suitably usable as MR head material for a recording density higher than 1 to 10 Gbit/inch.sup.2. However, these papers and publications merely disclose the basic action of the spin valve film.
Ni.sub.0.8 Fe.sub.0.2 (Permalloy) is mainly used as the MR head material for actual ultrahigh density magnetic recording at present. This material converts the change of a signal magnetic field from a magnetic recording medium into the change of electrical resistance by utilizing an anisotropic magnetoresistance effect. The MR ratio is in the range of 1 to 3% at most. In this case, the magnetoresistance change has a characteristic which is symmetrical at increasing and decreasing sides of magnetic field with the magnetic field of zero at the center.
As a means of solving this characteristic, in case of NiFe, etc., a shunt layer of Ti or the like which has a low resistivity is provided to shift an operating point. Furthermore, in addition to the shunt layer, a soft film bias layer which is formed of soft magnetic material having a large resistivity such as CoZrMo, NiFeRh or the like is also provided to apply a bias magnetic field. However, the structure having such a bias layer complicates its manufacturing process, and makes it difficult to stabilize its characteristics, resulting in cost-up. Furthermore, in this case, a gently-sloping portion of an MR change curve which is caused by the shift of the MR curve is used, and thus the MR slope per unit magnetic field is reduced to a small value of about 0.05%/Oe, resulting in reduction of S/N. Therefore, this value is insufficient as the MR head material for the recording density higher than 1 to 10 Gbit/inch.sup.2.
Furthermore, in case of MR heads, etc., there are some cases where a laminate structure is complicated, and thermal treatments such as baking, curing, etc. of resist materials are required in a patterning process, a flattening process, etc., so that heat resistance against a temperature of about 300.degree. C. is required for MR materials. However, such a thermal treatment deteriorates the characteristics of the conventional artificial superlattice structure.
With respect to the conventional spin valve film as disclosed in the papers, etc., only the basic structure and basic characteristics thereof as a thin film are argued, and any MR head structure to realize the ultrahigh density recording and any magnetic multilayer structure suitable therefor are not described.
As described above, in the examples as described in these papers, when the thin films as described in these papers are applied as an MR head, the MR slope is small in an actual magnetic field detection range, and thus an excellent and stable reproduced output cannot be obtained by the MR head. Furthermore, an MR change curve at an applied magnetic field of -10 to 10 Oe is important as a more excellent MR head material in the ultrahigh density magnetic recording. However, any of these papers has no argument on the details of the MR slope in this range.
Furthermore, a high-density recording and reproducing MR head is required to be used under high-frequency magnetic field above 1 MHz. However, in the film-thickness structure of each type conventional three-element magnetic multilayer, it is difficult to set above 0.2%/Oe of the slope (MR slope at a high frequency) of a magnetoresistance change curve at a width of 10 Oe in the high frequency magnetic field range above 1 MHz to obtain high sensitivity at high frequencies.